Obesity is defined as the result of excessive intake of food combined with reduced, or lack of physical activity. Although there are obese people who eat and move normally, and their fatness is enzymatic and / or genetically conditioned. Today, obesity is epidemic, even in the Mediterranean area countries known for the acclaimed Mediterranean diet. The causes of obesity are still explored by the scientific community, and are classified as exogenous and endogenous. A lot is known about exogenous causes, in the first place there are excessive eating and physical inactivity. Endogenous causes are less explored, although genetic basis are emphasized. A variety of methods is used for the diagnosis of obesity and body fat percentage. The simplest among them are extremely precise anthropometric measurements such as body mass index, waist circumference, the ratio of waist and hips, determining skin fold,and percentile curves for children and adolescents. Obesity is the cause of various chronic diseases, like the cardiovascular diseases which are a leading cause of death in Croatia and in the world. Obesity prevention is extremely important. The most successful prevention of child obesity is in older preschool period, and the younger school age. Diet therapy of overweight and obesity is the leading treatment. There are certain surgical measures and medications that are used today, however, they turn out to be a high risk for the patient because of their side effects, and the return to the old weight is very common regardless of the application. Another important factor in overweight and obesity treating is physical activity because it starts the metabolism, prevents certain diseases, strengthens the body and consumes the excess kilo calories. The nurse / technician plays an important role as part of the interdisciplinary team in the prevention and treatment of overweight and obesity.